The turbine housing of a steam turbine generally comprises an inner housing and an outer housing, an interspace being formed between the inner housing and the outer housing. These two housing parts in turn have an upper half and a lower half; in the case of high-pressure turbines the outer housing is also embodied as a barrel-type design. In particular after shutdown of the steam turbine there appear, at and between the housings, temperature differences between the lower half and the comparatively hot upper half which can be several degrees Kelvin.
If the steam turbine is shut down, the outer housing cools faster than the inner housing. As a consequence of free or natural convection, this induces an upward flow in the interspace between the inner housing and the outer housing which causes heat to be introduced into the upper half of the outer housing. This, in turn, may lead to distortion of the housing, particularly in the upper half of the outer housing, with the result that, there, undesirable stresses arise in the housing material and clearances are closed. Distortion of the inner housing can lead to undesirable rubbing-induced damage if, in adverse situations, turbine blades rub against the housing.
Steam turbines generally have an inner housing and an outer housing which surrounds the inner housing, with this double casing housing construction forming an interspace. The inner housing is at least partly encased, in its axial extent, by a cladding arranged in the interspace.
After a steam turbine is shut down or has shed load, there is a certain quantity of steam in the space between the inner and outer housing, depending on the prevailing pressure. Natural convection gives rise to temperature stratification between the upper and lower regions in the housing or, as the case may be, in the interspace. These different temperatures lead to distortion of the outer housing, which is also termed bowing. This “bowing” is to be avoided as the inner housing rests on the outer housing, inside the latter, and the distortion can cause the inner housing to be misaligned with respect to the rotor, which, at worst, results in closure of the radial clearances and possibly in rubbing.
Until now, this problem has been solved by providing a sufficiently large radial clearance. However, this causes a deterioration in efficiency.